In a color television system, the luminance and chrominance components of a composite video signal may be separated from each other by means of a digital comb filter. To improve the quality of received pictures, it is desirable to enhance vertical transitions (details) of the luminance signal derived from the output of the comb filter. Vertical detail signals to be enhanced are generated by element-by-element comparing the luminance data on the current horizontal line of a video signal shown in FIG. 1 with the luminance data on the horizontal lines immediately preceding and following it. The vertical detail values are increasingly positive on bright-to-dark transitions between top and bottom lines. Conversely, they are increasingly negative on dark-to-bright transitions between top and bottom lines.
FIG. 2a illustrates basic enhancement of the vertical detail signal, and FIG. 2b shows a block-diagram of the circuit that implements the basic vertical detail enhancement. As depicted in FIG. 2b, a comb filter 42 filters a composite video signal to separate luminance and chrominance components. A vertical detail processing unit 44 processes the previous, current and following horizontal lines shown in FIG. 1 to generate vertical detail signals. To supply the processing unit with the data in the three successive lines, the composite video signal is delayed by periods equal to the duration of one and two horizontal lines, i.e. 63.55 ms and 127.10 ms, respectively, for the NTSC standard, using delay units 46, each of which provides delay by one line width. The resultant vertical detail signal is amplified by a variable-gain amplifier 48 and is added by an adder 50 to the luminance output of the comb filter 42 to generate the enhanced luminance signal.
The adjustable slope of the transfer characteristic in FIG. 2a defined by the gain of the amplifier 48 determines the amount of enhancement per unit of vertical detail magnitude in both positive and negative directions. To eliminate the enhancement of noise, small amplitude excursions of the luminance signal may be removed by means of a process commonly referred to as "coring". This may be accomplished by means of a circuit for modifying the vertical detail signals having a transfer characteristic shown in FIG. 3, wherein no amplification is provided in a low amplitude region of the vertical detail signal (coring region). Further, gain above a certain level is either undesirable or unrequired. Accordingly, the transfer characteristic in FIG. 3 may be modified to restrict amplification in a high level gain region (paring region).
As shown in FIG. 4a, while small noise is cored out, larger noise that exceeds the coring level is amplified. To reduce the enhanced noise amount, a transfer curve shown in FIG. 4b should be generated, wherein a slower slope is provided in a transition region 64 between the coring region 62 and active region 66.
Accordingly, it would be desirable to provide a digital vertical detail enhancement system having the transfer characteristic with a lower slope in the transition region to reduce amplification of noise exceeding the coring level.
Further, it would be desirable to provide an adjustable coring level to suppress noise at different predicted levels and to provide an adjustable paring level and width of the transition region to customize the transfer characteristic.